FIG. 11 is a block diagram schematically illustrating a typical conventional photoreceptor for an optical fiber, for use in digital audio device. The photoreceptor is provided with a photodiode PD11 for receiving an optical signal transmitted via the optical fiber, and a signal processing circuit for performing signal processing in accordance with output from the photodiode PD11. The photodiode PD11 generates an electronic signal that is in proportion with intensity of the optical signal. A photocurrent detected by the photodiode PD11 is linearly converted into a voltage by a current-to-voltage conversion amplifier including an amplifier AMP 11, a resistor Rf11, and a capacitor Cf11. A bypass filter including a capacitor C11 and a resistor R11 removes a low frequency component from the voltage that has been converted from the photocurrent. Then, a constant voltage source Vref gives an operating point is given to the voltage. After that, the voltage is supplied to an amplifier circuit AMP13.
Moreover, a capacitor Cpd11 having the same capacitance as a parasite capacitance caused between the photodiode PD11 and GND is connected to an current-to-voltage converting amplifier including an amplifier AMP 12, a resistor Rf12, and a capacitor Cf12. A current supplied from the capacitance Cf12 is converted into a voltage by the current-to-voltage converting amplifier. Further, a bypass filter including the capacitor C12 and a resistor 12 removes a low frequency component from the voltage that has been converted from the current supplied from the capacitance Cf12. Then, the constant voltage source Vref gives an operating point to the voltage. The voltage is supplied to the amplifier circuit AMP13 so that the voltage is differential with respect to output of the amplifier AMP11.
With this circuit configuration, a common-mode noise component such as power source line noise and the like, is removed by a differential circuit.
The output of the amplifier circuit AMP 13 is supplied to a hysteresis comparator COMP 11 so that a waveform of the output is shaped. Then, an output stage 11 converts the output into a high-low digital signal, and outputs the high-low digital signal. The output stage 11 is composed of an inverter provided with an Nch (N channel) MOS (Metal Oxide Semiconductor) transistor MN11 and a Pch (P channel) MOS transistor MP11.
The photodiode PD11 and the signal processing circuit is integrated in a single chip. The photoreceptor integrated in a single chip is bonded on a wire frame by using a conductive adhesive material such as a silver paste, and wire-bonded with gold wires, so that a power source line and an output terminal are connected to terminals of the wire frame. Then, a lens is formed from a transparent mold resin, so as to be located on the photodiode PD11, whereby the photodiode PD11 and the lens are integrally structured.
In the photoreceptor that performs such digital output, as shown in FIG. 12, a parasite current of the photodiode flows in synchronism with rise and fall of the output, due to capacitive coupling between a wire connected to the output terminal, and the photodiode. The parasite current causes erroneous operation, thereby deteriorating sensitivity in receiving the optical signal.
There is a case the integration of the photodiode and the signal processing circuit in a single chip results in a larger parasite capacitance between the photodiode and the output terminal, compared with a photoreceptor including a photodiode and a signal processing circuit, which are formed in separate chips. More specifically, when the configuration in which a photoreceptor includes a photodiode and a signal processing circuit formed in separate chips, is compared with the configuration in which a photoreceptor includes the photodiode and the signal processing circuit integrated in a single chip, the output terminal and the photodiode are located closer to each other in the latter configuration. Thus, the latter configuration has a larger parasite capacitance between the output terminal and the photodiode. This is because the parasite capacitance is in inverse proportion with a distance between two neighboring conductive elements where the two neighboring conductive elements have the same area.
In order to solve this, contrived is a configuration in which a dummy photodiode is provided instead of the capacitor Cpd11, the dummy photodiode having the same area as the photodiode. The dummy photodiode has substantially the same area as the photodiode. A wire for cathode potential covers the dummy photodiode from light. Thus, even if an optical signal is introduced into the dummy photodiode, no current signal is outputted from the dummy photodiode. In this configuration, the parasite current from the output terminal can flow into the dummy photodiode and the photodiode evenly. In this case, a common-mode signal component is removed by the differential amplifier. This reduces erroneous operation.
However, in case where the dummy photodiode having the same area as the photodiode in the same chip in which the photodiode and the signal processing circuit are provided, a photodiode area (area that the dummy photodiode and the photodiode occupy) is doubled, thereby increasing chip area. It is a problem that this is disadvantageous in terms of cost. This problem is significant especially in a photo acceptance IC for digital audio optical fiber having a larger photodiode area than photo acceptance IC for photocoupler and optical disc.
Moreover, even in the configuration in which the dummy photodiode and the photodiode have the same area, there are some cases that the dummy photodiode and the photodiode have different parasite capacitances due to (i) difference in how the gold wire is extended from the output terminal, (ii) difference in location of the dummy photodiode and the photodiode, and (iii) other differences. In other words, if the parasite capacitance caused between the output terminal and the dummy photodiode becomes imbalanced with the parasite capacitance caused between the output terminal and the photodiode because of the difference in how the gold wire is extended from the output terminal, and the difference in the location of the dummy photodiode and the photodiode, this imbalance causes erroneous operation, too.
On the other hand, it may be arranged such that a transparent conductive film such as an ITO (Indium Tin Oxide) film is so provided on the photodiode as to cover the photodiode, the transparent conductive film being connected to a GND potential associated with a PD11 (that is, associated with the portion of the circuit for receiving the optical signal). In this arrangement, a noise current caused by a capacitance caused between input terminal and output terminal flows into the GND by the transparent conductive film. Thus, only an optical signal from the photodiode is outputted. This prevents erroneous operation caused by noises. However, it is necessary to have an apparatus specially for covering the photodiode with the ITO film. This complicates manufacturing process of the photoreceptor. Moreover, the large parasite capacitance caused between the ITO film and the photodiode lowers band of an amplifier of the receptor and increase noises, thereby retarding high speed communication.
Next, a current to be fed back from the output terminal to the photodiode is discussed below. The current Ip to be fed back form the output terminal to the photodiode is given by the following equation:Ip=Cp·(dV/dt)  (1),where Cp is the parasite capacitance caused between the output terminal and the photodiode, and (dV/dt) is a slew rate of the rise or the fall of the output voltage.
At time t, the rise of output voltage has a waveform given by the following equation:V=Vo(1−exp((t/Rout·Cout))))  (2),where Rout is an output resistance of an output stage, Cout is a capacitance of the output stage, and Vo is a amplitude of the output voltage.
Because the output stage has a cutoff frequency fo=1/(2π·Rout·Cout), Equation (2) is:V=Vo(1−exp(−2π·fo·f))  (3).
Thus, the slew rate (dV/dt) of the output can be represented by the following equation:(dV/dt)=Vo·2π·fo·exp(−2π·fo·t)  (4).
Therefore, by substitution of Equation (4) in Equation (1), the current Ip to be fed back from the output terminal to the photodiode at the rise of the output is:Ip=Cp·Vo·2π·fo·exp(−2π·fo·t)  (5).FIG. 13(a) illustrates an output waveform worked out by Equation (5) with an assumption that the parasite capacitance Cp between the output terminal and the photodiode is 10 fF, rise time tr in which an output amplitude of the output is increased from 10% to 90% is 10 ns, and the output amplitude Vo is 3V.
fo=0.35/tr, where the output starts to rise when t=0. Thus, from Equation (5), the parasite current Ip caused at the rise of the output at time t is:Ip=(10 fF)·(3V)·2π·(0.35/10 ns)·exp(−2π·(0.35/10 ns)·  (6).The fall of the output causes a parasite current that is identical in size but opposite in a flow direction with respect to the parasite current Ip caused at the rise of the output. From Equation (6), Ip has its peak when t=0.
The parasite pulse caused by the parasite capacitance caused between the output terminal and the photodiode has a parasite current waveform as shown in FIG. 13(b), for example. Let t=0 in Equation (6), then the peak of Ip is:Ip=(10 fF)·(3V)·2π·(0.35/10 ns)=6.6 μA  (7).That is, a peak current is 6.6 μA.
An optical signal for digital audio device is transmitted at 5.6448 Mbps in case of equi-speed, at 11.2896 Mbps in case of double speed, and at 22.5792 Mbps in case of four-time speed. For transmitting such signal, the acceptance circuit needs to have a wider band of amplifier for the transmission carried out at a faster speed. An effect of the parasite current pulse is larger in case the amplifier has a wider band.
The following explains the band of the amplifier. Generally, a gain of the amplifier has such a frequency characteristics that the gain of the amplifier is lower at a higher frequency. A frequency at which the gain of the amplifier changes to a gain of −3 dB is called as a cut-off frequency (fc). Generally, the band of the amplifier refers to the cut-off frequency. That is, it is necessary to increase a frequency of a signal for transmitting the signal at a higher speed, and it is necessary to increase the band of the amplifier in order to sufficiently amplify the signal.
Note that all of the parasite current obtained from Equation (6) does not get amplified thereby causing no erroneous operation, because the waveform of the parasite current includes a high frequency component. If the bond of the amplifier of the photoreceptor is set to be 0.8 times of the transmission speed (Mbps), the band of the amplifier is 4.5 MHz in case of equi-speed, 9 MHz in case of double speed, and 18 MHz in case of four-time speed. If a pulse waveform of the parasite photocurrent given by Equation (6) is passed through low-pass filters respectively having cut-off frequencies of 4.5 MHz, 9 MHz, and 18 MHz, circuit simulation shows that peak currents are 0.627 μA, 1.602 μA, and 1.686 μA. These currents are amplified and causes the erroneous operation.
The receptor for the digital audio device linked with an optical fiber has a minimum reception sensitivity in a range of from −27 dBm to −24 dBm. At the minimum reception sensitivity, a photodiode current that flows in accordance with a signal is approximately 0.5 μA to 1 μA. That is, the effect of the parasite current between the output terminal and the photodiode is not negligibly small.
Moreover, if the band of the amplifier is wider, a signal of a higher frequency component can be amplified. Thus, if the band of the amplifier is wider, the high-frequency peak current of the parasite current is amplified. In other words, because a wider band of the amplifier causes a larger parasite current, the parasite current pulse causes erroneous operation more often in the receptor having a higher transmission speed.
On the other hand, when the power source voltage is increased, an ON-resistance of the inverter including the Nch MOS transistor and Pch MOS transistor in the output stage 11 is decreased thereby decreasing output resistance. The following explains this.
A drain current ID of the MOS transistor is given by:ID=K(Vgs−Vt)2(1+λVds)  (8),where K is transconductance coefficient, Vt is a threshold voltage, Vgs is a gate-source voltage, Vds is a drain-source voltage, and λ is a channel length modulation coefficient.
In case of the output stage is structured by forming an inverter including the Nch MOS transistor and the Pch MOS transistor, Vgs=Vds=Vcc (power source voltage). Thus, the drain current is given by:ID=K(Vcc−Vt)2(1+λVcc)  (9).Accordingly, a resistance R of the MOS transistors, which is represented as Vcc/ID, is given by:R=Vcc/ID=Vcc/(K(Vcc−Vt)2(1+λVcc))  (10).Equation 10 shows that the increase in power source voltage decreases the output resistance R.
As described above, the increase in the power source voltage decreases the output resistance of the output stage 11, whereby the rise time tr and the fall time tf become earlier and output amplification is enlarged. Thus, the value of (dV/dt) in the parasite current I=C×(dV/dt) is increased. As a result, possibility of the erroneous operation is increased. Moreover, even if it is adjusted such that the erroneous operation will not occur at a power source voltage of 3V, periods of tr and tf are shortened at the power source voltage of 5V. This results in an increase in the output amplitude that leads to a large parasite current. As a result, the erroneous operation occurs. Moreover, if tr and tf are adjusted such that the erroneous operation will not occur at a power source voltage of 5V, tr and tf are delayed when the power source voltage becomes 3V. This limits a transmission speed at which the signal can be transmitted. This makes it difficult to realize a high-speed digital output receptor having a wide range of operational power source voltage.
Moreover, Publication of Japanese Patent No. 3018541 (published on Mar. 13, 2000) discloses an output circuit whose slew rate of an output is two-stage switched ON in accordance with a slew rate control signal supplied from outside. This output circuit is incapable of performing a fine control of the slew rate within a range in which the erroneous operation can be inhibited, because the control of the slew rate of the output is only two-staged.